In general, it is desirable that lasers be compact in size while, at the same time, producing a high power, high quality output beam. Unfortunately, in many laser systems such desired requirements cannot all be fulfilled simultaneously, so design compromises must be made.
The characteristics of the output beam depend on the distribution of the light inside the optical resonator of the laser. The light inside an optical resonator is distributed in well defined patterns termed modes.
The optical quality of the output beam from an optical resonator is determined by its divergence, which differs for each mode within the resonator. The highest quality output beam has the least divergence.
A parameter which provides a quantitative measure for the output beam is called M.sup.2. This parameter is described in A. E. Siegman, New developments in laser resonators, SPIE Vol. 1224, 2-14 (1990), the disclosure of which is hereby incorporated by reference.
The optimal shape of the output beam with the least divergence is the Gaussian shape, where M.sup.2 =1. Output beams with larger M.sup.2 parameters have greater divergence, so their resulting beam quality is inferior. If only a single mode is present inside the resonator, then the output beam from the resonator can be improved by placing outside the optical resonator a compensating optical element, designed especially to improve the divergence for that mode.
The field distribution of the modes affects the power of the beam emerging from the resonator. One of the most common ways to describe this field distribution is using a cylindrical representation as described in the following publications:
A. G. Fox and T. Li, Resonant modes in an optical maser, Bell Sys. Tech. J., Vol. 40, 453-488 (1961).
H. Kogelnik and T. Li, Laser beams and resonators, Proc. IEEE, Vol 54, No 10, 1312-1328 (1966).
The field distribution is expressed as follows: EQU F.sup.NM (r,.THETA.)=R.sup.NM (r)*Exp(iN.THETA.) [1]
where r is the radial coordinate, .THETA. is the angular coordinate, N is the angular factor describing the angular distribution of the mode, M is the radial factor describing the radial distribution of the mode and R could be an arbitrary complete set of orthogonal functions.
There are known several techniques to 1) change the field distribution of the modes, 2) achieve mode discrimination and 3) increase the output power. These techniques are briefly described hereinbelow:
1) Mode shaping: The field distribution of the modes is mainly determined by the reflectors and the lenses that are incorporated into the optical resonator. Typically, the shape of these elements is spherical, and the field distribution of the resulting modes is well known as described, in H. Kogelnik and T. Li, Laser beams and resonators, Proc. IEEE, Vol 54, No 10, 1312-1328 (1966). PA1 2) Mode discrimination: The number of modes in an optical resonator can be reduced, even to a single mode, by introducing an aperture inside the resonator. A parameter that describes the relative aperture width is called the Fresnel number. The Fresnel number is defined by the following expression: ##EQU1## where a is the radius of the aperture, L is the length of the optical resonator and .lambda. is the wavelength of the mode. PA1 3) Increasing output power: The gain medium inside the laser resonator must be illuminated by the field distribution of the mode in an optimal way in order to achieve maximal output power. For example, an annular shape of illumination is best for a DC discharge CO.sub.2 laser where a high temperature along the longitudinal axis of the laser reduces power of the center of the output beam. The conventional way to obtain such a field illumination shape is to use optical resonators with large Fresnel numbers, corresponding to wide apertures.
Recently, new types of reflectors have been introduced in order to control the field distribution of the modes. These reflectors are independent of angular coordinates but dependent on radial coordinate. Consequently, they can control only the radial field distribution of the modes. Reflectors of this type are described in P. A. Belanger, P. L. Lachance and C. Pare, Super-Gaussian output from a CO.sub.2 laser by using a graded phase mirror resonator, Opt. Let, Vol. 17, No. 10, 739-741 (1992).
When the Fresnel number is small, corresponding to a small aperture, only the mode with the narrowest field distribution propagates while the rest of the modes suffer a large increase in loss of intensity and cease to exist. In such a situation, single mode operation can be easily achieved, thereby discriminating it from the other modes. The quality of the resulting output beam is high, with corresponding relatively low M.sup.2. Unfortunately, such a configuration, having a small internal aperture, is not suitable for many laser resonators, as explained hereinbelow.
As a result of using wide apertures, many modes exist simultaneously, so that a large portion of the gain medium is illuminated. This method for increasing the output power results in a reduction of the beam quality. This is because an output beam having modes with a wide field distribution diverges strongly as it emerges from the laser. Due to the fact that more than one mode exists simultaneously, the use of a compensating optical element to improve the output beam quality is not possible.
Techniques have been proposed to change the angular distribution of the output beam after it emerges from the laser, in order to reduce divergence. Reference is made to L. W. Casperson, N. K. Kincheloe and O. M. Stafsudd, Phase plates for laser beam compensation, Opt. Comm., Vol. 21, No. 1, 1-4 (1977). Unfortunately, these techniques for changing the angular distribution do not reduce the divergence as defined by the M.sup.2 parameter. Reference is made to A. E. Siegman, Binary phase plates cannot improve laser beam quality, Opt. Lett., Vol. 18, No., 675-677 (1993).
In summary, it is appreciated that in conventional optical laser resonators a compromise must be made between increased output power and output beam quality. Expressed in more technical terms, this means that the need for wide optical resonators with large Fresnel numbers is contradicted by the need for output beam with low divergence, i.e. low M.sup.2.